2013 review tech qd-leds klimov

8/12/2019 2013 REVIEW Tech QD-LEDs Klimov

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721 2013 Materials Research Society MRS BULLETIN VOLUME 38SEPTEMBER 2013 www.mrs.org/bulletin

Introduction: Traditional and emerging LED

technologiesBeginning with the ancient myth of Prometheus, who stole fire

from Mount Olympus, to third millennium technologies, the

ability to master artificial light incarnates the very concept of

progress and modernity. Throughout history, the impact of

artificial lighting on human civilization has been so profound

that it has arguably redefined our concepts of time and space,

extending our productive time and allowing humankind to

explore even the darkest corners of our world.

Our society relies so heavily on lighting that it dedicates a

fifth of global electricity to it, which amounts to over 2,650 TWh

of energy. In 2010, the environmental cost associated with

the production of this amount of energy was equal to abouttwo billion tons of carbon dioxide emitted in the atmosphere.1

Unfortunately, most of this precious energy is wasted due to

the inefficiency of incandescent bulbs that convert only 10%

of electricity into visible light, while 90% is dissipated as heat.

Fluorescent lamps are less wasteful but still have a conversion

efficiency of only 2030%. The economic advantage of over-

coming these limitations, together with a growing awareness

of the harmful effects of carbon emission on global climate,

has driven the tremendous effort to develop more efficient andsustainable lighting technologies.

Direct conversion of electricity into light using semicon-

ductor-based light-emitting diodes (LEDs) is now universally

accepted as the most promising approach to more efficient

lighting. Solid-state lighting (SSL) sources such as LEDs

demonstrate high brightness, long operational lifetime, and low

energy consumption, far surpassing the performance of conven-

tional lighting systems. The LED field is currently dominated by

semiconductor quantum-well emitters (based, e.g., on InGaN/

GaN) fabricated via epitaxial methods on crystalline sub-

strates (e.g., sapphire).1 These structures are highly efficient

and bright, but their cost-to-light-output ratio (also referredto as the cost of light) is at least one-hundred times larger

than that of incandescent light bulbs. In addition, traditional

semiconductor LEDs are affected by structural defects at the

substrate/semiconductor interface due to lattice mismatch and

heating during operation, which limits the device area to only

12 mm2. Furthermore, harsh conditions required for the fab-

rication of these structures and difficulties in post-processing

Spectroscopic insights into theperformance of quantum dotlight-emitting diodesWan Ki Bae, Sergio Brovelli, and Victor I. Klimov

Lighting consumes almost one-fth of all electricity generated today. In principle, with more

efcient light sources replacing incandescent lamps, this demand can be reduced at least

twofold. A dramatic improvement in lighting efciency is possible by replacing traditional

incandescent bulbs with light-emitting diodes (LEDs) in which current is directly converted

into photons via the process of electroluminescence. The focus of this article is on the emerging

technology of LEDs that use solution-processed semiconductor quantum dots (QDs) as light

emitters. QDs are nano-sized semiconductor particles whose emission color can be tuned by

simply changing their dimensions. They feature near-unity emission quantum yields and narrow

emission bands, which result in excellent color purity. Here, we review spectroscopic studies

of QDs that address the problem of nonradiative carrier losses in QD-LEDs and approaches

for its mitigation via the appropriate design of QD emitters. An important conclusion of our

studies is that the realization of high-performance LEDs might require a new generation of

QDs that in addition to being efcient single-exciton emitters would also show high emission

efciency in the multicarrier regime.

Wan Ki Bae, Chemistry Division, Los Alamos National Laboratory; [email protected] Brovelli, Department of Materials Science, Universit degli Studi di Milano-Bicocca; [email protected] I. Klimov, Chemistry Division, Los Alamos National Laboratory; [email protected]

DOI: 10.1557/mrs.2013.182


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SPECTROSCOPIC INSIGHTS INTO THE PERFORMANCE OF QUANTUM DOT LIGHT-EMITTING DIODES

722 MRS BULLETIN VOLUME 38SEPTEMBER 2013 www.mrs.org/bulletin

complicate their use in lightweight, flexible lighting devices and

displays. These drawbacks of traditional LEDs have motivated

the search for new technologies, such as LEDs based on

organic semiconductors (OLEDs).

OLEDs offer greatly reduced fabrication cost compared to

inorganic LEDs and are easily amendable to low-temperature,

large-area processing, including fabrication on flexible sub-

strates. Synthetic organic chemistry provides essentially an

unlimited number of degrees of freedom for tailoring molecu-

lar properties to achieve specific functionality, from selective

charge transport to color-tunable light emission. The prospect

of high-quality lighting sources based on inexpensive plastic

materials has driven a tremendous amount of research in the

area of OLEDs, which in turn has led to the realization of

several OLED-based high-tech products such as flat screen

televisions and mobile communication devices. Several industri-

al giants such as Samsung, LG, Sony, and Panasonic are work-

ing on the development of large-area white-emitting OLEDs

for replacing incandescence light bulbs and fluorescent lightsources.

In the most common embodiment, an OLED has a multilayer

structure comprising an emissive layer (typically an organic

matrix doped with molecular chromophores) sandwiched

between organic hole and electron transport layers.2,3 The

emissive chromophores are usually either conjugated fluo-

rophores or phosphorescent metal-organic complexes.4,5

Conjugated fluorophores (e.g., 4-(Dicyanomethylene)-2-methyl-

6-[p-(dimethylamino)styryl]-4H-pyran, coumarins and perylene

derivatives)3 have been a major subject of OLED research for

over two decades.68 They feature the high emission yields

and short luminescence lifetimes typical of singlet emitters,which can convert the energy of singlet excitons (electron

hole pairs with total spin 0) to photons. However, their draw-

back is that their maximum electroluminescence (EL) internal

quantum efficiency (IQE; the ratio of the number of photons

emitted within a device to the number of electrons passing

through the device) is limited to 25%, as 75% of excitons gen-

erated by charge injection are triplets, which are nonemissive

spin 1 species. To date, external quantum efficiencies (EQEs;

the ratio of the number of photons emitted from a device to the

number of electrons passing through the device) of over 10%

have been obtained with red9,10 and green11 conjugated fluo-

rophores that also demonstrate impressive device lifetimes inexcess of 150,000 hours. Blue OLEDs are more difficult to

realize, and so far the demonstrated EQEs from fluorescent

emitters do not exceed 56%, and the longest lifetimes are

about 10,000 hours.12

The more recently introduced phosphorescent metal-organic

complexes13 allow one, in principle, to reach 100% IQE by over-

coming the limits imposed by branching between spin states of

different multiplicities. Red and green OLEDs based on phos-

phorescent iridium complexes showed EQEs above 20% with

a turn-on voltage below 3 V.14 More recently, green-emitting

OLEDs with an optimized light out-coupling design demonstrat-

ed an EQE of 63%.

15

Blue phosphorescent OLEDs also exhibit

performance competing with that of the best devices based on

blue fluorescent molecules, exhibiting EQEs as high as 14%.16

Despite impressive advances in the OLED field, there are

a few drawbacks of this technology that might prevent its

widespread use in commercial products. One problem is poor

cost-efficiency, which is at least partially due to the complex-

ity of the necessary device architecture, which requires pro-

cessing involving multiple thermal deposition steps. Another

problem is their limited stability, particularly for deep-red and

blue phosphorescent OLEDs. While being greatly improved

over the years, it still does not meet the standards employed

in high-end devices. Furthermore, the color purity of OLEDs,

that is, the width of the emission band, is inferior to that of

conventional semiconductor LEDs. Specifically, the emission

full width at half maximum of OLEDs is typically greater than

40 nm (Figure 1, dashed lines), while it is less than 30 nm for

conventional LEDs.

A promising class of emissive materials for low-cost yet

efficient LEDs is chemically synthesized nanocrystal quantumdots (QDs). These luminescent nanomaterials feature size-

controlled tunable emission wavelengths, combined with

improvements in both color purity (Figure 1, solid lines) as

well as stability and durability over organic molecules. At the

same time, as with organic materials, colloidal QDs can be

fabricated and processed via inexpensive solution-based tech-

niques compatible with lightweight, flexible substrates.1719

Similar to other semiconductor materials, colloidal QDs

feature almost continuous above-band-edge absorption and a

narrow emission spectrum at a near-band-edge energy. Distinct

from bulk semiconductors, however, the optical spectra of QDs

depend directly on their size. Specifically, their emission colorcan be continuously tuned from the infrared (IR) to ultraviolet

Figure 1. High color quality of quantum dot (QD) emission.

Comparison of typical electroluminescent spectra of blue, green,

and red QD-LEDs (light-emitting diodes) (solid lines) to those of

OLEDs emitting at similar wavelengths (dashed lines).


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to dense QD films typically used as the active layers of LEDs.

In this case, the role of surface recombination is amplified be-

cause of energy transfer (ET), which allows for a given exciton

to sample trap centers in multiple QDs within the ET distance

(the distance for which the ET time is equal to the exci-

ton recombination time in an isolated QD) (Figure 2b). In

QDs, ET typically leads to a red shift of the PL peak, which

is accompanied by acceleration of PL decay, especially pro-

nounced on the blue side of the emission spectrum, which cor-

responds to smaller dots in the sample.52,53 The emitting dipole

of these dots readily couples to the strong absorption dipole

of bigger dots, which leads to a migration of excitons to the

largest QDs in the ensemble.

For closely spaced CdSe QDs (e.g., in dense Langmuir

Blodgett films54), the ET time can be as short as ca. 100 ps.

It is slower in commonly used films made via drop cast-

ing, dip coating, or spin coating; however, even in this case,

the characteristic time of ET (typically a few nanoseconds52)

is faster than the radiative lifetime (20 ns for CdSe QDs55).Therefore, it can compete with radiative recombination, lead-

ing to reduced PL QYs compared to solution samples.

To quantify this reduction, we consider a simplified situ-

ation where the QD ensemble comprises only two types of

dots: bright with PL QY equal to unity and dark with a

zero QY. The corresponding QY (Q0) of a non-interacting QD

ensemble is simply defined by the fraction of bright dots. If

this sample is assembled in a film, the exciton generated in a

bright QD can either recombine within it (radiatively under

our assumptions) or migrate to a different dot with the total

rate kET. Since the acceptor dot can be nonemissive (dark)

with a probability (1Q0), each ET step leads to the reduction inthe PL QY. For the infinite number of these steps, the overall

PL QY can be found from

( )

( ) ( ) }0 r r ET

2

0 ET r ET 0 ET r ET1 ,

k k k

k k k k k k

= +

+ + + + +

Q Q

Q Q (1)

which yields

( ( )0 r r ET 01 .k k k = + Q Q Q (2)

If, for example, we consider a sample with Q0= 0.8 and typi-cal values of radiative (0.05 ns1) and ET (0.5 ns1) rates,

we obtain Q = 0.27, three times lower than the original

QY. This estimation shows that even highly luminescent

solution samples may experience a significant reduction in

effective emission efficiency upon processing into a film.

This highlights the need for designing QD samples that fea-

ture not only high emission efficiencies in solution form,

but also suppressed ET when they are assembled into dense

films.

A recent study of LEDs based on CdSe/CdS QDs56 has

demonstrated that an increase in the shell thickness (H) leads

to a progressive increase in the LED EQE, which among other

factors was attributed to suppression of exciton quenching by

ET. Figure 2c displays the effect of increasing shell thickness

on PL dynamics of QD film versus solution samples. As was

pointed out earlier, the PL decay measured on the blue side

of the emission band of the QD films represents a qualitative

measure of the ET time. As evident from Figure 2c, this

decay becomes progressively slower with increasing H, and

in the thickest-shell sample (H= 6.4 nm), it is essentially the

same as in the solution of non-interacting QDs. This suggests

a nearly complete suppression of ET due to the thick shells

acting as spacers separating the cores of the donor and the

acceptor QDs involved in the ET process. The observed ET

suppression is well correlated with the increase in the EL EQE

of QD films incorporated into LEDs.56

Hot carrier trapping/recombination and B-type QDblinkingMore recently, there has been increasing evidence from both

single-dot57,58 and ensemble5961 measurements of the importanceof nonradiative recombination involving surface/interface trap-

ping of hot, unrelaxed carriers (Figure 3a). In the absence

of other nonradiative channels, the PL QY due to hot electron

trapping/recombination can be described by Q= kcool/(kcool+ kesc),

where kescis the rate of hot-electron escape from the QD, and

kcoolis the rate of intraband relaxation to the emitting band-

edge state. A distinct feature of this mechanism is that while

leading to reduced PL intensity, it does notchange the emis-

sion lifetime, which under the previous assumption is defined

by radiative decay of a band-edge exciton. If both channels

(band-edge and hot-carrier trapping) are active, the resulting

QY can be expressed as Q= kcoolkr/[(kcool+ khot)(kr+ knr)].Hot-carrier trapping has been invoked to explain photocharg-

ing of QDs under excitation with above-bandgap photons,5961

and recently was used to explain one of the regimes (so-called

B-type blinking) of PL intermittency (that is, switching of

a QD between high- and low-emissivity states) observed

in single-dot measurements.58 This regime is characterized by

strong fluctuations in PL intensity that are not accompanied

by any significant variations in PL lifetime. This is exactly

the signature of a recombination channel due to hot-electron

traps, which randomly turns on and off due to fluctuations in

the traps occupancy.58

Strong support for this model was provided by demonstra-tions of the control of B-type blinking by applied potential for

QDs incorporated into an electrochemical cell.58 Specifically,

it was observed that under positive bias, which corresponded

to a lower Fermi level, the low-emissivity (OFF) periods were

extended due to increasing time spent by the hot-electron trap

in the unoccupied, active state (Figure 3b). On the other hand,

under sufficiently negative potential, the B-type OFF periods

were completely suppressed, as expected for the situation

of a higher Fermi level, which leads to filling (that is, blocking)

of hot-electron traps (Figure 3cd). This explanation of B-type

blinking suppression due to filling of electron-accepting trap sites

is consistent with previous observations of PL enhancement


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725MRS BULLETINVOLUME 38SEPTEMBER 2013www.mrs.org/bulletin

and blinking suppression due to electron-donating molecules62,63

and n-doped substrates.64

As with band-edge trapping, the B-type hot-electron trap-

ping can be suppressed by growing a sufficiently thick shell.

The analysis of blinking behaviors for CdSe/CdS QDs58 indi-

cates that the occurrence of B-type blinking is reduced as the

thickness of the CdS shell is increased, and it is completely

eliminated in samples with shells comprising more than 15

monolayers of CdS. These thick-shell QDs (often referred to

as giant- or g-QDs) were either nonblinking or exhibited

so-called A-type blinking due to charging/discharging of the

QD core.58

Interestingly, a closer inspection of lifetime tra-jectories indicated that some of the nonblinking dots, while

showing stable emission intensity, were still characterized by

significant fluctuations of the PL lifetime (lifetime blinking57)

due to switching between neutral exciton and negative trion

states; the charged state was highly emissive because of sup-

pressed Auger recombination in this type of structure (as dis-

cussed in the following sections).

Nonradiative Auger recombination of chargedexcitonsAn important mechanism for nonradiative losses in QDs

is associated with Auger recombination of multicarrier states,

that is, states involving more than one exciton.

The simplest of these states are singly charged

excitons, or negative (Figure 4a) and positive

(Figure 4b) trions (Xand X+, respectively).

In addition to radiative or extrinsic defect-

related processes, charged excitons can decay

via nonradiative Auger recombination, a pro-

cess in which the energy of an electronhole

pair is converted not into a photon but instead

is transferred to a third carrier. This process

is inefficient in bulk forms of wide-gap semi-

conductors, but due to relaxation of momen-

tum conservation,65 it is extremely efficient

in QDs, where it is characterized by a short

(a few ps to hundreds of ps) time constant

(A) that exhibits direct scaling with QD vol-

ume (V), A V, sometimes referred to as

V-scaling.66,67

Charged states can be created within QDsunder light exposure (photocharging5961), due

to direct electrical contact with a metal or

semiconductor substrate, or can arise because

of an imbalance between electron and hole

injection currents in LED devices. There is

ample evidence that charging has a significant

detrimental impact on the performances of

light-emitting systems relying on QDs.41,58,6782

To quantify the effect of extra charges on QD

emission, we assume that the radiative decay

rate scales linearly with the number of available

recombination pathways, that is, it is directlyproportional to the product of the electron (Ne) and the hole

(Nh) occupancy of the QD:83,84

( )r e h e h 1r ; ,k N N N N k = (3)

where k1ris the radiative decay rate of a neutral exciton (Ne= 1;

Nh= 1). Based on this expression, the radiative rates of nega-

tive and positive trions are k+1r= k1r= 2k1and corresponding

PL QYs are

( ) ( )1 1 1 1A 1 1 1 1A2 2 and 2 2 . + += + = +k k k k k k Q Q (4)

Since in standard core-only or thin-shell QDs the rate of Auger

decay is several orders of magnitude higher than the radiative

decay rate, the expression for QYs can be approximated by

Q1 2k1/k1Aand Q+1 2k1/k+1A.

The trion Auger lifetimes can be inferred from spectroscopic

measurements of a neutral biexciton (a state comprising two

electrons and two holes) decay rate (k2A), which is related to

the Xand X+decay rates as k2A= 2(k1A+ k+1A). In core-only

QDs that feature mirror symmetric conduction and valence

bands, k1A= k+1A= k2A/4. Such a situation is realized, for

example, in PbSe QDs for which the observed trion Auger

lifetime is indeed approximately four times longer than that of

Figure 3. Hot-carrier recombination centers and B-type quantum dot (QD) blinking.58

(a) Carrier trapping by hot-electron (or hole) traps (D1; rate kesc) competes with intraband

relaxation into the emitting band-edge state (ratekcool); this process is only active if the D1trap

is unoccupied, that is, the Fermi level (FL) is below the trap level. (b) Hot electron trapping

can lead to so-called B-type blinking when the uctuations in the photoluminescence (PL)

intensity (black trace) are not accompanied by appreciable changes in the PL lifetime

(red trace). The intensity and lifetime trajectories are shown for CdSe/CdS QDs (shell

thickness is 3.6 nm) incorporated into an electrochemical cell. These specic data were

collected for the positive potential of 0.8 V, which shifts the FL down in energy and

increases the time spent by the D1trap in the unoccupied state. (c) The hot-electron

trapping channel can be shut off by raising the FL, which leads to lling of the D1traps.

(d) At sufciently high negative potential (1 V in the gure), the B-type blinking is completely

suppressed.


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a biexciton (2A). Recent measurements of chemically doped

samples85 indicate that in PbSe QDs with Eg = 0.76 eV

(R= 3.85 nm), the XAuger lifetime is 360 ps, very close tothe expected value of 416 ps predicted from the measured 2A

of 104 ps.

The symmetry between the Xand X+recombination path-

ways can be distorted in QDs of IIVI semiconductors due to

the significant difference between the conduction and valence

bands. Specifically, a much higher density of valence-band

states favors the positive trion Auger recombination channel

(Figure 4b) for which the energy conservation requirement is

easier to meet. Recent measurements86 have evaluated the

biexciton and negative trion Auger lifetimes in thin-shell CdSe/

ZnS QDs using time-resolved picosecond cathodolumines-

cence. The results of these measurements are summarized inFigure 4c (+1Ais derived from the measured 1Aand 2A). One

can see that in larger dots (Eg= 1.94 eV), +1Aand 1Aare close

to each other (770 ps and 799 ps, respectively). However, as

the dot size is decreased, the XAuger pathway gets progres-

sively suppressed due to increasing separation between the

conduction band states, so that in QDs with Eg= 2.34 eV, the

Auger lifetime of the positive trion (126 ps) is ca. twice as fast

as that of the negative trion (254 ps).

The asymmetry between the Xand X+recombination path-

ways is further enhanced in coreshell structures that feature

a significant difference in effective localization radii (defined

by the spatial extent of electronic wave functions) of electrons

(Re) and holes (Rh). One example is thick-shell

CdSe/CdS g-QDs.70,75 In these dots, the electron

is delocalized over the entire nanostructure,

while the hole is confined to the smaller core,

and hence, Re is much greater than Rh. This

enhances the Auger decay rate of the positive

versus negative trion due to increased prob-

ability of hole-hole scattering events (involved

in the X+decay channel; Figure 4b) compared

to electron-electron scattering (involved in the

Xdecay channel; Figure 4a). For example,

based on a recent experimental study of core

shell CdSe/CdS QDs,68 in samples with core

radius r= 1.5 nm and H= 5.5 nm, 1A= 5 ns

and 2A= 0.35 ns (inset of Figure 4d). These

values yield +1A = 0.81 ns, which suggests

a very significant asymmetry between the X

and X+Auger decay channels with 1A 6+1A.

On the basis of the previous analysis of trionAuger lifetimes, we can estimate their emis-

sion efficiencies. Using the data obtained for

standard thin-shell CdSe/ZnS QDs and assum-

ing a 20 ns neutral-exciton radiative time con-

stant,55 we estimate that for the sample with

Eg= 2.34 eV (Figure 4c), the negative trion PL

QY is only 2.5%, and this is even lower (1.3%)

for positive trions. The trion QY is increased

for larger samples (e.g., Q1 Q+1 7% for

QDs with Eg= 1.94 eV); however, it is still below 10%.

These estimations indicate that QD charging can greatly

reduce emission efficiency and hence the EQE of LEDs. Aswas shown in previous work,72,73,82 the use of g-QDs with

an ultra-thick CdS shell allows for significant suppression of

Auger recombination, which translates into increased trion

lifetimes and hence emission yields; this was cited as one of

the reasons for improved performance of LEDs comprising

CdSe/CdS g-QDs.56 Indeed, for the thick-shell CdSe/CdS

sample in Figure 4d, we estimate that the negative trion PL

QY is 20%, which is considerably higher than in standard

dots. However, we would like to point out that because of a

significant asymmetry between the Xand X+Auger pathways

in these structures, the X+QY is still low (3%) and is com-

parable to that in standard QDs. This observation highlightsthe fact that while charging is in general detrimental to LED

performance, the presence of extra holes is especially harm-

ful, as positive trion lifetimes in many types of IIVI QDs can

be significantly shorter than the lifetimes of negative trions.

Suppression of Auger recombination via interfaceengineeringThe previous analysis indicates that Auger recombination

represents a serious impediment to the realization of high

emission efficiency in situations where the formation of multi-

carrier states is difficult to avoid, such as high-intensity (and/or

high-photon energy) optical excitation or electrical pumping

Figure 4. Auger recombination and the effect of interfacial alloying on the Auger decay

rate. (a) Auger recombination of a negative trion can be described in terms of Coulombscattering between the two conduction band electrons when one of them undergoes an

interband transition, while the other is excited within the conduction band. (b) Similarly,

Auger recombination of a positive trion can be described as Coulomb scattering between

the two valence-band holes. (c) Auger lifetimes of biexcitons (2A), and negative (1A)

and positive (+1A) trions in standard thin-shell CdSe/ZnS quantum dots (QDs) with

different bandgaps (Eg). (d) Early time Auger dynamics of mult icarrier states for reference

coreshell (C/S) CdSe/CdS QDs with an abrupt interface and core/alloy/shell (C/A/S) CdSe/

CdSexS1x/CdS QDs with an alloyed interface (xis ca. 0.5). Both samples have the same core

radius (r= 1.5 nm) and the same total radius (R= 7 nm). The thickness of the interfacial

alloy is 1.5 nm.68 (e) Schematic illustration of synthesis of the C/S and C/A/S QD samples.


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when even a slight imbalance in electron and hole currents

can lead to QD charging with extra carriers. Therefore, the

development of approaches for suppression of Auger decay

could greatly benefit LEDs.

As was mentioned earlier, one approach to reducing Auger

decay rates involves overcoating CdSe cores with exception-

ally thick CdS shells.72,73 Originally, these structures were

expected to exhibit suppressed Auger recombination based on

V-scaling arguments and the reduced electronhole overlap

due to a considerable mismatch in effective radii of electron

and hole localization (Re>> Rh). However, as was pointed out

in Reference 72, these factors alone could not explain the obser-

vation of very rapid lengthening of 2Awith increasing shell

thickness, suggesting that other structural characteristics of

the QD such as the properties of its interfaces might play an

important role in Auger recombination.

Indeed, calculations by Cragg and Efros71 as well as a

follow-up theoretical study by Climente et al.87 have sug-

gested that the shape of the confinement potential also has asignificant effect on the Auger decay rate. Specifically, these

studies demonstrated that by smoothing the confinement

potential (via, e.g., alloying the interfacial layer), one can

reduce the overlap between the initial and the final state of the

carrier excited during Auger recombination and thus achieve

an orders-of-magnitude reduction in the rate of this process.

Recently, the problem of the effect of inter-

facial alloying on Auger recombination was

directly addressed by studying carrier dynam-

ics in coreshell CdSe/CdS QDs with a well-

defined CdSexS1x alloy incorporated between

the core and the shell (Figure 4de).68,88 Thesestudies were enabled by a novel synthesis

method,68 allowing much faster growth of a

thick CdS shell compared to a more traditional

successive ionic layer adsorption and reaction

(SILAR) method,70,75,89 which was essential for

avoiding unintentional alloying at the CdSe/

CdS interface. Using this method, it was pos-

sible to synthesize structures with either a

sharp coreshell boundary (C/S QDs) or with a

CdSexS1x alloy layer incorporated between the

core and shell (C/A/S QDs); see Figure 4e.68

Side-by-side studies of spectroscopic prop-erties of these QDs68 indicated that the incor-

poration of the interfacial alloy layer did not

significantly influence either spectra or dynamics

of single excitons; however, it had a profound

effect on multicarrier dynamics and PL QY

of multicarrier states. For example, a direct

comparison of the C/S and C/A/S samples

(alloy layer thickness L= 1.5 nm and x 0.5)

with the same core radius (r= 1.5 nm) and the

same total size (R= 7 nm) indicates that the

Auger lifetimes are systematically longer for

the C/A/S QDs (Figure 4d). While observed

for both Xand X+states, lifetime lengthening is especially

pronounced for positive trions (2.8 ns versus 0.81 ns for the

C/A/S and the C/S samples, respectively), which results in

considerable improvement of PL QY (12% versus 3% in

the sample without the interfacial alloy). A stronger effect of

interfacial alloying on X+dynamics is expected based on the

fact that it primarily affects the shape of the hole confinement

potential. As discussed in the next section, the use of alloyed

dots in LEDs allows one to improve their EQE compared to

coreshell structures with a sharp interface, and this improve-

ment is especially pronounced for high injection currents when

the EQE loss due to Auger decay is particularly significant.

Effects of charging in LEDs and high-intensityroll-off of LED effi ciencyWhen QDs are incorporated into devices, they can acquire a

net charge due to interactions with electrodes and/or charge

transport layers. Recently, this effect has been studied81 using

the so-called inverted LED architecture, where a QD activelayer is sandwiched between an inorganic metal-oxide elec-

tron transport layer and an organic hole transport layer based

on conjugated molecules (Figure 5a). This architecture has

been previously applied in high-performance devices2729,81,90

that feature low turn-on voltage (nearly the same as the

QD bandgap), high external EQEs at practical brightness

Figure 5. An inverted quantum dot light-emitting diode (QD-LED) with organic

and inorganic charge transport layers and the inuence of QD charging on device

performance.81 (a) Schematic illustration of the structure of an inverted QD-LED with

inorganic and organic charge transport layers. Electrons are injected from the ZnO/

ITO side, and holes are injected from the CBP/MoOx/Al s ide.28 (b) An energy band

diagram of a device with CdSe/CdSexS1x/CdS QDs forming an active emitting layer.

(c) Photoluminescence (PL) decay dynamics of QDs in solution, on glass, and within

the device. Inset: PL lifetimes for QDs within the device versus applied voltage. (d)

External quantum efciencies (EQEs) versus current density for two QD-LEDs: one

employing CdSe/CdS QDs with an abrupt in terface (black diamonds) and the other,

CdSe/CdSexS1x/CdS QDs (xis ca. 0.5), with an alloyed interface (red circles). Both samples

have the same core radius (r= 1.5 nm) and the same total radius (R= 7 nm); the thickness

of the CdSexS1x alloy layer is 1.5 nm.


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(1001000 cd/m2), and good operational stability (half life-

time 500 hr).28,29

Figure 5b shows an energy diagram of a device utilizing

alloyed CdSe/CdSexS1x/CdS QD emitters (same samples as

in Figure 4de) and an n-type colloidal ZnO electron injec-

tion layer assembled on an indium-tin-oxide (ITO) cathode

together with a 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP) hole

conducting layer separated from the Al anode by a thin MoOxspacer.81 Due to the energetic proximity of the QD conduction-

band level to the Fermi level of the ITO cathode, electron

injection from ITO into QDs occurs spontaneously at zero bias,

as indicated by a direct comparison of PL dynamics for dots

assembled on the passive glass substrate versus the dots on

the ITO/ZnO electrode (Figure 5c). In the former case, the PL

dynamics are nearly the same as for isolated QDs in solution,

while in the latter, the dynamics are significantly faster due to

Auger recombination of the negative trion state arising from

the combination of a photoexcited exciton and an injected elec-

tron. Indeed, the measured PL lifetime of 8.5 ns is very close tothe Auger recombination time of a negative trion inferred from

spectroscopic studies of QD solutions (inset of Figure 4d).

The fact that the observed changes in carrier decay are due

to excess electrons is confirmed by the observed changes in

the PL lifetime as a function of applied bias (inset of Figure 5c).

Under reverse bias, which makes electron injection less

favorable, the PL decay becomes slower as expected for the

situation in which a QD fluctuating between the charged and

the neutral state spends an increasing amount of time in the

neutral state.58 On the other hand, the PL dynamics get faster

under direct bias due to increasing contribution from doubly

negatively charged excitons.58

A distinct asymmetry in the response to negative versus

positive bias also indicates that the observed changes in PL

dynamics are not significantly altered by effects that are in-

dependent of the electric-field direction, such as field-induced

spatial separation between a photo-generated electron and a

hole with a QD.90 These observations also lead

to the important conclusion that a likely reason

for the efficiency roll-off (also known as effi-

ciency droop) at large currents is the increasing

degree of dot charging with excess electrons,

which leads to increasing nonradiative carrier

losses due to Auger recombination. Indeed, ifwe compare LEDs fabricated using coreshell

CdSe/CdS QDs to those made using alloyed

CdSe/CdSexS1x/CdS dots, which exhibit sup-

pressed Auger recombination (Figure 4de),

we observe that the onset of the EQE roll-off

is considerably higher in alloyed QDs than

in structures with a sharp coreshell interface

(Figure 5d).

Summary and outlookQD-LEDs have advanced tremendously over

the past two decades, and progress has been

especially fast recently due to significant improvements in the

quality of QD materials as well as advances in the architec-

ture of the devices. As illustrated in Figure 6, over the years,

the QD-LED architecture has evolved from proof-of principle

polymer-QD bilayer structures to modern devices employing

direct charge injection from finely tuned electron and hole

transport layers (see, e.g., Figure 5a). Recent refinements of

this latter architecture have led to the demonstration of devices

with a peak EQE of 18%,29 which is near the theoretical maxi-

mum of 20%. At the same time, parallel engineering efforts

have demonstrated the feasibility of full-color active-matrix-

driven QD displays with patterned red, green, and blue QDs

active layers.27

As in the field of OLEDs, an important outstanding challenge

in the field of QD-LEDs is the stability of device operation,

especially for high injection currents. For example, the record

efficiency devices of Reference 29 were able to maintain the

highest EQE for only less than one hour. The reasons for

the instability likely relate to both chemical (photochemical)degradation of the QDs as well as photophysical effects.

As discussed in the previous sections, growing evidence

suggests a detrimental effect of charging on LED perfor-

mance. The presence of excess charges in the QDs might lead

to both reversible degradation of the LED efficiency due to

Auger recombination as well as irreversible changes due to

activation of chemical reactions at the QD surface and/or in

the ligand shell.41 As discussed earlier, charging is also a likely

reason for the significant efficiency roll-off observed for high

driving currents.28,29,81

These considerations imply that realization of high-

performance LEDs requires QD materials that exhibit highemission efficiencies not only in the single-exciton but also

multi-carrier regimes. Two examples of such structures dis-

cussed in this article are thick-shell CdSe/CdS g-QDs70,73,75

and recently demonstrated CdSe/CdSexS1x/CdS QDs, with

an intermediate alloyed layer introduced for suppression of

Figure 6. History of quantum dot light-emitting diodes (QD-LEDs). The rst reported

QD-LEDs were based on a QD/polymer bilayer structure.23After this demonstration of

proof-of-principle devices, the performance of QD-LEDs has been persistently improved

by employing various types of multilayer architectures with carefully engineered charge

transport layers (CTLs). The most notable examples are QD-LEDs with QD active layers

sandwiched between all-organic,92 all-inorganic,93 and hybrid organic/inorganic CTLs.94

In addition to rapid advances in the efciency and durability of individual QD-LEDs, recent

technological developments have yielded active matrix driven full-color QD displays with

patterned red, green, and blue QDs active layers.27


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Figure 7. Operational principle and schematics of an InGaN/GaN light-emitting diode

(LED)-backlit display with quantum dot (QD) color converters. (a) In a backlit display,

blue emission from an InGaN/GaN quantum well (QW) LED is partially absorbed by

green- and red-emitting QD down-converters; the mixture of three emitted colors results

in trichromatic white emission. (b) A schematic diagram of a liquid-crystal display utilizing

trichromatic QD-based backlight.

Auger recombination.68 Other reported structures that dem-

onstrated suppressed Auger decay include CdxZn1xSe/ZnSe78

and CdTe/CdSe.91

The detrimental effects of charging can also be reduced via

a careful choice/design of charge transport layers for achiev-

ing balanced electron and hole injection currents. Electrode

engineering can also help lower the operational voltage, and,

as a result, the strength of the electric field within the QD

layer. This might also help mediate the problem of efficiency

roll-off, as some studies suggest that in certain cases, this roll-

off can be attributed to the reduction of the QD emission rate

due to field-induced spatial separation between electrons and

holes.90

While the performance of QD-LEDs is quickly approaching

industry standards for lighting and displays, the first commercialapplications of QDs will likely utilize them as down-converters.

In such devices (e.g., backlit units), QDs are not excited by

passing current, but instead are excited optically by absorb-

ing blue radiation from InGaN/GaN quantum well LEDs

before reemitting green or red light (Figure 7ab).3034 Recently,

Sony Corporation has announced that it plans to incorporate

QD-based down-conversion white LEDs as a backlight unit

into their LCD television sets. Similar directions are being

pursued by other big companies such as Samsung, LG, etc.

Given these recent advances, it is reasonable to expect that the

very exciting moment when the first QD product appears on

the market is just around the corner.

AcknowledgmentsThis work was supported by the Chemical Sciences, Biosciences,

and Geosciences Division of Office of Science, Office of

Basic Energy Sciences, US Department of Energy.

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